This application claims the benefit of Japanese Patent Applications No. 2001-103731 filed Apr. 2, 2001, No. 2001-103746 filed Apr. 2, 2001, No. 2001-103762 filed Apr. 2, 2001, No. 2001-103776 filed Apr. 2, 2001, and No. 2001-278494 filed Sep. 13, 2001, in the Japanese Patent Office, the disclosures of which are hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to signal processing methods and signal processing apparatuses, and more particularly to a signal processing method which is suited for carrying out a predetermined signal processing with respect to a light detection signal that is obtained by detecting light received via an information recording medium in an information recording and/or reproducing apparatus such as a CD-ROM drive apparatus, CD-R drive apparatus, CD-RW drive apparatus, DVD-ROM drive apparatus, DVD-R drive apparatus, DVD-RW drive apparatus, DVD-RAM drive apparatus, MD drive apparatus and MO drive apparatus, and to a signal processing apparatus which employs such a signal processing method. CD refers to compact disk, DVD refers to digital versatile disk, MD refers to magnetic disk, and MO refers to magneto-optical disk.
2. Description of the Related Art
In an optical disk drive apparatus, for example, a laser beam is irradiated on an optical information recording medium such as an optical disk, and the laser beam reflected by the optical disk is received and detected by an optical pickup. More particularly, the reflected laser beam is detected by a light receiving element such as a photodetector (PD). A light detection current is output from the photodetector and is supplied to a signal processing unit. Generally, the light detection current is a weak current. Hence, the light detection current is usually subjected to a current-to-voltage conversion so as to generate a light detection signal, and the light detection signal is supplied to the signal processing unit. A high signal-to-noise (S/N) ratio is required of the light detection signal, so as to guarantee a high reproduction quality and a stable servo operation.
An amount of light received by the photodetector and a servo error signal gain vary due to inconsistencies in the amount of light irradiated on the optical disk, inconsistencies in the reflectivity of the optical disk, and inconsistencies in the characteristics of the individual optical pickups. In order to compensate for such inconsistencies, a gain adjustment is made in each signal processing unit.
The optical pickup is moved in a radial direction of the optical disk during a so-called seek operation. Hence, the optical pickup and a circuit board on which the signal processing unit and the like are mounted are, in general, connected by a flexible printed circuit (FPC) substrate which can be bent freely, so as not to interfere with the movement of the optical pickup. A transmission path for the light detection signal is provided on this FPC substrate.
Various kinds of optical disks have been proposed in recent years. In addition, information recording and/or reproducing apparatuses which are operable with different kinds of optical disks having mutually different formats have also been proposed, so as to achieve optical disk compatibility among the information recording and/or reproducing apparatuses to a certain extent. Furthermore, various optical pickups have also been proposed for use in such information recording and/or reproducing apparatuses which achieve optical disk compatibility. There are demands to reduce the size and cost and to improve the performance of such optical pickups.
However, in the conventional information recording and/or reproducing apparatuses, the structure and circuit characteristics required of the signal processing unit differ for each of the optical pickups having the different structures corresponding to the different kinds of optical disks. Particularly, the structure of a processing section for generating a servo error signal differs for each optical pickup corresponding to each kind of optical disk. For this reason, it is necessary to design a signal processing unit (or signal processing integrated circuit) for each optical pickup corresponding to each kind of optical disk. As a result, there are problems in that it is difficult to reduce the time required to design and develop the signal processing unit and that it is difficult to reduce the cost of the signal processing unit.
Moreover, in the information recording and/or reproducing apparatus which is operable with the different kinds of optical disks having the mutually different media formats to achieve the optical disk compatibility, a plurality of kinds of signal processing units must be provided if the different kinds of optical disks require different structures and circuit characteristics of the signal processing unit. Consequently, the need to provide a plurality of signal processing units makes it difficult to reduce the size of the signal processing apparatus of the information recording and/or reproducing apparatus.
On the other hand, when an attempt is made to increase as much as possible a common part which may be used in common for each of the plurality of signal processing units, the specifications required of the optical pickup become extremely severe, thereby making the structure of the optical pickup complex and expensive. For this reason, it is difficult from the practical point of view to provide a large common part which may be used in common for each of the plurality of signal processing units.
Accordingly, a method of eliminating the above described problems was proposed in a Japanese Laid-Open Patent Application No. 11-238245, for example. According to this proposed method, the optical pickup converts the light detection current into the light detection signal by carrying out the current-to-voltage conversion, and this light detection signal is subjected to an analog-to-digital (A/D) conversion. Processes which are carried out thereafter, such as a process of extracting a reproduced RF signal and a process of computing an RF matrix for the various servo operations, are realized by a digital signal processing. Moreover, an operation process for generating a tracking error signal, switching of various servo gains, and a switching of a passband of a bandpass filter when extracting an address from a wobbled groove of the optical disk, are carried out depending on the media format of the optical disk. As a result, it is possible to realize a signal processing apparatus which carries out all of the various processes by the digital signal processing using a relatively simple structure.
However, according to the signal processing apparatus employing this previously proposed method, a processing section (or operation unit) must be provided with respect to each of the required servo error signals. In addition, in order to obtain the servo error signals which are computed by different operations due to the differences among the media formats of the optical disks, it is not only necessary to provide different processing sections for the servo error signals depending on the media format, but also to switch and use one of the processing sections depending on the media format of the optical disk in use. Consequently, there is a problem in that it is difficult to further reduce the size of the signal processing apparatus. Furthermore, if the structure of the optical pickup is modified and the computations for obtaining the servo error signal needs to be changed in accordance with the modification, there is another problem in that a circuit modification must be made in the signal processing apparatus depending on the structural modification of the optical pickup.